1. Field of the Invention
This invention relates to a focal length changeable lens with its exit pupil located at a long distance for use with a solid-state imaging device equipped with on-chip micro-lenses, and, more particularly, to a positive power focal length changeable lens positioned in front of a solid-state imaging device equipped with on-chip micro-lenses so as to effectively intake light into respective solid-state imaging elements of the imaging device.
2. Description of Related Art
With demands for high image quality, it is a recent tendency to form still pictures with a large number of picture elements. According to such demands, still video cameras are equipped with solid-state imaging devices, each device being comprised of a considerably large number of solid-state imaging elements. As the number of solid-state imaging elements per imaging device increases, each single solid-state imaging element is minimized, being made smaller and smaller. In addition, since the practical photosensitive area available as a light sensing means is only a part of the entire surface of the solid-state imaging element, the solid-state imaging device, comprised of a large number of such solid-state imaging elements, has the disadvantage that the light detected thereby is lessened. For this reason, it has been proposed to locate micro-lenses, such as convex lenses, with each micro-lens having an effective aperture larger than the practical photosensitive area, one for every solid-state imaging element, in order to direct light to the practical photosensitive area of each solid-state imaging element, thereby increasing the photosensitivity of the solid-state imaging device.
A reference is made to FIG. 5 for the purpose of providing a brief background of an on-chip micro-lens equipped solid-state imaging device that will enhance an understanding of the operation of the focal length changeable lens system of the present invention. As shown in FIG. 5, a solid-state imaging device 100 includes an imaging element array 101 of a large number of solid-state imaging elements, and an on-chip micro-lens array 102, of the same number of micro lenses as the solid-state imaging elements, placed over the imaging element array 101. Specifically, the photosensitive areas of the solid-state imaging elements is practically defined by apertures 103 which are isolated from one another. Each aperture 103 is precisely covered by a micro-lens element 104, such as a convergent or convex lens. All of these micro-lenses 104 are integrally formed in a single plate of the on-chip micro-lens array 102. Light rays impinging upon the micro-lenses 104 within their aperture 105 are diverged within the respective apertures 103 and focused on the surfaces of the solid-state imaging array 101, so as to form an image with high brightness. This is highly contributive to the sensitiveness of the solid-state imaging device.
Meanwhile, a focal length changeable lens system which is changeable in focal length between two focal lengths, such as a relatively long focal length and a relatively short focal length, is typically comprised of, in order from the object side, a first lens group, a diaphragm or aperture and a second lens group. The objective lens system is effected by the first lens group and the aperture with the second lens group is removed out of the optical path for providing a long focal length. On the other hand, it is effected by the second lens group and the aperture with the first lens group removed out of the optical path for providing a short focal length. Since such a focal length changeable lens system has been designed for use with still video cameras with no on-chip micro-lens equipped solid-state imaging devices, it unavoidably has an exit pupil at a long distance due to design limitations of these still video cameras.
However, if such a focal length changeable lens system, which is not designed and adapted so as to suitably cooperate with still video cameras with the on-chip micro-lens equipped solid-state imaging devices described above, is used, angles of marginal light rays incident upon micro-lenses is large relative to the direction parallel to the optical axes of the micro-lenses. Consequently, the amount of light impinging on each area of the photosensitive area defined by the aperture 103 is greatly reduced with an accompanying decrease in sensitivity of the solid-state imaging device.